In general, vacuum glazing is composed of a minimum of two glass panes separated by a void space with a thickness in the range starting at 100 μm and up to 800 μm. Sealing is obtained by a peripheral seal. To achieve super-insulation performances (coefficient of surface transmission U<0.6 w/m2K), the vacuum level between the glass panes must be in the order of 10−3 mbar or less, and generally at least one of the two glass panes must be covered by a low-emissivity layer having an emissivity of ideally less than 0.05.
Different seal technologies exist and each has some disadvantages. A first type of seal (the most widespread) is a seal based on a welding glass, the melting temperature of which is lower than that of the glass of the glazing panes. The use of this type of seal limits the choice of low-emissivity layers to those that are not impaired by the thermal cycle necessary for usage of the welding glass, i.e. to that which is resistant to a temperature that can be up to 350° C. Moreover, since this type of seal based on welding glass has very low deformability, it does not allow absorption of the effects of differential expansions between the glass pane of the glazing on the internal side and the glass pane of the glazing on the external side when these are subjected to substantial differences in temperature (e.g. 40° C.). Quite significant stresses are thus generated on the periphery of the glazing and can cause breakages of the glass panes of the glazing.
A second type of seal comprises a metal seal, e.g. a metal strip with a low thickness (<500 μm) welded around the periphery of the glazing by means of an attachment sub-layer covered at least partially with a layer of a solderable material such as a tin alloy soft solder. A significant advantage of this second type of seal over the first type of seal is that it can be deformed to absorb the differential expansions created between the two glass panes.
Patent application US 2008/0245011 A1 discloses a method for manufacturing a vacuum insulated double glazing having a peripheral vacuum-tight edge connection by welding together two metal foil strips connected to the peripheral zones of each glass pane's inner glazing face, the strips protruding beyond the edges of the glass panes. Welding is performed by sweeping a laser beam along the protruding parts of the metal foil strips.
Such glazing does not allow edge connections which do not extend outside the periphery of the panes. Additionally, after being welded together, the protruding parts of the foil strips must be bended onto the glass pane edges and mechanical weaknesses may be introduced in the bending areas.
Another disadvantage of the glazing according to US 2008/0245011 A1 is that mechanical stresses may still take place during the welding operation which is performed one zone at a time as the laser beam sweeps along.